1. Field of the Invention
The present invention relates to tape cartridges having two hubs with tape wound around each of the hubs and extending from one hub to the next, and more particularly to a belt-driven tape cartridge.
2. Description of the Related Art
The belt-driven tape cartridge, or data cartridge, has been known for many years, e.g., as shown in U.S. Pat. No. 3,692,255 (von Behren) and Japanese Laid-Open Patent Publication No. 48-15517. As shown in FIGS. 1-3 of the von Behren reference, the typical data cartridge includes a housing enclosing a pair of rotatably mounted tape hubs about which tape is wound. In the course of passing from one hub to the other, the tape passes along a predefined tape path in the data cartridge, which takes it past a cut-away portion through which the head of a drive can access the tape. The data cartridge housing also encloses a pair of corner rollers, and a drive roller. An elastic belt is stretched around the rollers and the tape packs on the hubs. The drive roller is accessible by a drive puck of a tape drive through an opening formed at the front of the housing. Movement of the drive roller by the puck causes the belt to move, and frictional engagement between the belt and the tape then causes the tape to move. In addition to moving the tape, the belt provides proper tension in the tape. The tape can be moved and read in either direction, i.e., either hub can serve as the supply hub and either as the take-up hub. The terms "supply hub" and "take-up hub" will be used in describing the operation of the invention herein, but it is to be understood that either hub could play the role of a supply hub or a take-up hub, depending on the direction of movement of the tape at any given moment.
Proper tension must be maintained in the tape for it to maintain proper contact with the head. As explained in the von Behren reference, this tape tension is created due to the combined effects of differences in speed between the tape hubs, the relative elasticity of the belt and inelasticity of the tape, and friction in various components in the data cartridge. Thus, a number of elements in the cartridge can be modified to adjust the tape tension.
In Advanced Information Storage Systems, vol. 1,49, 1991, Robert A. von Behren and David P. Smith put forth a formula which approximated the various effects on tape tension. The article indicates the tension of the tape on a reading/writing magnetic head is expressed by the following equation: EQU Th={(T3-T2)+Ts+Mb.multidot.tb(1/D1-1/D2)}.multidot.(Multiplier Term)
where
Th=the tensile force of tape on the reading/writing magnetic head PA0 T3-T2=the rotational frictional forces of the corner rollers PA0 Ts=the rotational frictional force of the supply hub PA0 Mb=the elastic modulus of the drive belt PA0 tb=the thickness of drive belt PA0 D1=the outer diameter of the tape pack on the supply hub PA0 D2=the outer diameter of the tape pack on the take-up hub PA0 Multiplier Term=a term relating to the frictional force between the tape and tape guide.
As tape is wound from the supply hub to the take-up hub, the diameters D1 and D2 gradually change. As shown by the third term of the above equation, this means that the tape tension also changes. In particular, this creates a gradient in the tape tension as the tape is wound from one end of the tape to the other. More specifically, the tape is moved by the drive belt, which in turn is driven by an external motor via the drive roller. As the tape winds onto the take-up hub, the drive belt must stretch to accommodate the extra diameter. At the same time, the supply hub shrinks, allowing the drive belt to contract around it. The result is that the speed of the supply hub varies compared to that of the take-up hub. Since the tape is relatively inelastic compared to the belt, the difference between the speeds of the hubs is converted into tensile force in the tape and, consequently, the difference between the driven speeds of both tape packs increases as the difference in the pack diameter of the tape wound into tape packs around both hubs increases, thus generating a gradient in the tensile force of the tape.
In practice, the difference between the minimum and maximum tensile force on the tape typically is approximately 0.49 N. This difference causes several problems: (1) The contact pressure between the magnetic head and the tape varies with the tape tension, so that stable contact pressure cannot be maintained. (2) Variations in contact pressure in turn means that the tape may alternately stick and slip, i.e., stable traveling speeds may be difficult to achieve. (3) The edge of the tape must withstand a greater range of tension, which makes it more difficult to obtain high durability in the tape. (4) It is difficult to reduce errors in recording or reproducing information due to all of the foregoing variations. (5) The motor in the drive must be capable of driving the tape accurately at both the highest and the lowest drive force levels. The broader the range, the greater the difficulty this poses.
Designing a suitable system to use a data cartridge is complex. As will be apparent from the foregoing, it is necessary to be sure the maximum force needed to move and tension the tape is smaller than the maximum drive force which a drive can provide. It is important that the drive provide enough force to the cartridge to maintain the minimum tape tension necessary for the magnetic head to make good contact with the tape. It is also necessary to control frictional forces between various parts in the data cartridge. For example, it is necessary to minimize the frictional forces in the contact portions between the tape and tape guides, between the drive roller and its shaft, and between the tape hubs and their shafts. On the other hand, it is necessary for the corner rollers to maintain a certain level of friction to generate an appropriate drag force. Thus, it is not desirable to reduce the frictional force in the corner rollers as far as possible.
Japanese Laid-Open Patent Publications Nos. 63-191350 and 2-158977 propose certain techniques for adjusting the tensile force of the tape in a data cartridge. These include adjusting the outer diameters of the corner rollers, and setting the viscosity of grease applied between the corner rollers and their pivot pins appropriately.
U.S. Pat. No. 4,172,569 (Newell) teaches yet another technique. Newell's data cartridge includes, in addition to the drive belt of the conventional data cartridge, an endless belt called a tensioning belt. The tensioning belt is engaged on the drive roller and the corner rollers, but at a different height from the drive belt. Thus, the tensioning belt does not press directly or indirectly against the tape wound on the tape hubs.
The Newell data cartridge has several problems: (1) Since the drive belt and the tensioning belt are both engaged on the drive roller, a large load is applied directly to the drive roller. This increases the drive force required to move the tape, in turn causing the motor driving the cartridge to consume more power. (2) Since the two belts are engaged on the drive roller and the corner rollers at different levels, there is a possibility that the axes of the rollers will become eccentric. This would prevent the tape from traveling reliably, with the result that noise would be generated during travel of the tape. In extreme cases, the belt or tape might come off its guides. (3) Due to the increased space required for the tensioning belt, the Newell data cartridge is thicker than a standard data cartridge. This limits the ability to increase the recording capacity per unit volume.
European Patent Application No. 0 579 116 (Schoeppel et al.) teaches another technique, which is to adjust the k factor (stretchability) of the belt. Unfortunately, the extent to which one can adjust the k factor is somewhat limited by the materials used, typically polyurethane. Thus, while this may allow some reduction in the bow tie, it cannot eliminate it.
Finally, while some friction in the belt is essential, friction between the tape and the tape guides normally is not essential to operation of the data cartridge. For example, U.S. Pat. No. 5,358,193 (Madsen et al.) teaches the use of self-acting hydrodynamically lubricated guides, in which a layer of air minimizes friction between the corner tape guides and the tape.